1. Field of the Invention
This invention relates to a method of protecting the surface of the heater of an ink-jet recording head, an ink-jet recording apparatus, a recording unit and a method of prolonging the service life of an ink-jet recording head.
2. Related Background Art
Various techniques have been proposed for printing using an ink-jet recording system. For instance, Japanese Patent Application Laid-Open No. 54-51837 describes an ink-jet method (a so-called bubble jet method) of ejecting ink droplets under the effect of thermal energy. This method provides a remarkable advantage of producing high quality images at high speed and low cost on ordinary plain paper not carrying any special coat layer because the method allows to prepare a high density multi-nozzle head with ease. With this method, the heater of the recording head is rapidly heated to cause the liquid laid on the heater to generate a bubble and quickly increase its volume so that liquid droplets (ink droplets) are ejected from the nozzle that is arranged at the front end of the recording head under the effect of the quick voluminal expansion of the liquid. Then, the ejected ink droplets are forced to fly and hit the target recording medium for printing.
However, when ink of a certain type, or ink containing alkali metal ions (sodium ions, lithium ions or the like) to be more specific, is used with this bubble jet method to produce a large volume of printed paper, the surface of the heater (heat generating resistor) of the recording head can become dissolved. Therefore, it is of vital importance to suppress or prevent the phenomenon of a dissolved surface of the heater of the recording head in order to provide high quality images on a stable basis, prolong the service life of the recording head and consequently reduce the load on the part of the environment. Thus, it has been and still is a technological challenge to provide improved recording heads from the above described point of view.
Japanese Patent Application Laid-Open No. 5-320550 discloses an ink composition containing ammonium salt such as ammonium halogenide or ammonium sulfonate up to 0.2 wt % in an attempt to dissolve this problem. However, halogenide ions and sulfonic acids are strongly acidic and corrosive. Therefore, when ink having such a composition is contained in the recording apparatus for a long time, the areas of the apparatus held in contact with the ink can become corroded and the effect of preventing a surface of the head of the recording head from being dissolved (etched) can be significantly lowered.
U.S. Pat. No. 5,462,590 (Japanese Patent Application Laid-Open No. 6-220386) discloses an ink composition containing water, a coloring material and an amine compound expressed by the general formula shown below to be used for thermal ink-jet recording; 
(where any one or each of any two of R1, R2 and R3 represents an alkyl group with one to five carbon atoms substituted by a carboxyl group, a sulfonic acid group or the Li, Na, K or ammonium salt thereof and each of the remaining two or the remaining one, whichever appropriate, of them represents an alkyl group with one to five carbon atoms substituted by a hydrogen atom, a hydroxyl group or a carbamoyl group).
Specific examples of amine compounds that can be used for the above ink composition include N,N-bis(2-hydroxyethyl)-2-aminosulfonic acid, N,N-bis(2-hydroxyethyl)glycine and aminoethane sulfonic acid (taurine). However, U.S. Pat. No. 6,048,390 (Japanese Patent Application Laid-Open No. 10-36735), the applicant of which is same as that of the above identified patent, describes that the addition of primary amine as covered by the above U.S. Pat. No. 5,462,590 does not necessarily result in any noticeable improvement in terms of kogation and the primary amine reacts itself near the heater due to the rapid temperature rise and becomes insoluble to aggravate the kogation when a dye having a carboxyl group is used and the pH level is regulated to between 6 and 8. In short, the patent document describes that the use of primary amine is far from advantageous under certain conditions.
Ink and the related liquid composition can contain alkali metal ions produced from hydroxide of alkali metal (such as lithium hydroxide, sodium hydroxide or potassium hydroxide) and operating as counter ions relative to the carboxyl group or the sulfonic acid group in the molecule of the coloring material in order to prevent the nozzle of the ink-jet recording head from clogging. As a result of a series of studies of the inventors of the present invention, it was found that, when solution containing an alkali metal ion is ejected repeatedly from a bubble jet system, the metal and/or the metal oxide of the outermost surface protection layer of the heater can become dissolved and the heater can give rise to broken wires to make the bubble jet system no longer able to eject ink and the related ink composition. The inventors of the present invention presume that, as the liquid composition (including ink) containing an alkali metal ion is heated excessively by the heater, the concentration of the alkali metal ion rises near the heater to make the composition highly alkaline, which by turn dissolves the protection layer on the outermost surface that is made of metal such as tantalum and/or metal oxide.
On the basis of this technological presumption, the inventors of the present invention examined the effect of adding primary amine that is denied in the above identified patent document to ink containing an alkali metal ion to find that the behavior of the added primary amine is vitally affected by the rate at which energy is applied to the heater. In other words, it is possible to maximally suppress the phenomenon of scraping the heater of ink containing both primary amine having a structure as shown in formula (1) below and an alkali metal ion by controlling the rate at which energy is applied to the heater. Additionally, the side effect of raising the level of kogation due to the addition of primary amine can be suppressed. The inventors of the present invention also found that the effect of energy control is not dependent on the type of the coloring material of the ink. More specifically, the effect of the present invention was proved for ink containing a dye having a carboxyl group with a pH level regulated to between 6 and 8 for which U.S. Pat. No. 6,048,390 reports that any addition of primary amine does not necessarily result in any noticeable improvement in terms of kogation.
On the basis of the above finding, it is therefore an object of the present invention to provide a method of protecting a surface of a heater of an ink-jet recording head to be used for applying thermal energy to the ink in the recording head and causing the recording head to eject ink.
Another object of the present invention is to provide an ink-jet recording apparatus that can print images with improved image quality and operate with a prolonged service life.
Still another object of the present invention is to provide a recording unit having a prolonged service life for printing. A further object of the present invention is to provide a method of prolonging the service life of a recording head that can print images with improved image quality at reduced cost.
In an aspect of the present invention, there is provided a method of protecting an outermost surface protection layer of a heater of an ink-jet printer having a recording head, the outermost surface protection layer of the heater containing at least metal or oxide thereof, said heater being used for an ink-jet printing process comprising the step of applying energy (Eop) to the heater to eject the liquid composition from the recording head,
the liquid composition comprising a liquid medium, an alkali metal ion and a compound expressed by the following general formula (1):
H2Nxe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94Xxe2x80x83xe2x80x83(1),
X representing a carboxyl group or a sulfonic acid group, Y representing a hydrogen atom, a carboxyl group or a sulfonic acid group, n representing 0 or 1, m representing an integer from 0 to 2;
wherein the energy (Eop) satisfies the relation as defined by the following formula (2):
1.10xe2x89xa6Eop/Ethxe2x89xa61.90xe2x80x83xe2x80x83(2),
Eop representing the energy applied to the heater in order to cause the recording head to eject the liquid composition, Eth representing the minimum energy required by the recording head to eject the liquid composition.
In another aspect of the invention, there is provided an ink-jet recording apparatus comprising a liquid composition container containing a liquid composition, an ink-jet recording head having a heater for applying thermal energy to the liquid composition in a liquid composition flow path led out from the liquid composition container and a means for applying a pulse-shaped electric signal to said heater according to the information to be recorded, said heater having an outermost surface protection layer containing at least metal or oxide thereof;
said liquid composition comprising a liquid medium, an alkali metal ion and a compound expressed by the following general formula (1):
H2Nxe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94Xxe2x80x83xe2x80x83(1),
X representing a carboxyl group or a sulfonic acid group, Y representing a hydrogen atom, a carboxyl group or a sulfonic acid group, n representing 0 or 1, m representing an integer from 0 to 2;
said apparatus satisfying the relation as defined by the following formula (2):
1.10xe2x89xa6Eop/Ethxe2x89xa61.90xe2x80x83xe2x80x83(2),
Eop representing the energy applied to the heater in order to cause the recording head to eject the liquid composition, Eth representing the minimum energy required by the recording head to eject the liquid composition.
In still another aspect of the invention, there is provided a recording unit comprising a liquid composition container containing a liquid composition and an ink-jet recording head having an orifice for ejecting the liquid composition under the effect of thermal energy, said ink-jet recording head being provided with a heater for applying thermal energy to the liquid composition, said heater having an outermost surface protection layer containing at least metal or oxide thereof;
said liquid composition comprising a liquid medium, an alkali metal ion and a compound expressed by the following general formula (1)
H2Nxe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94Xxe2x80x83xe2x80x83(1),
X representing a carboxyl group or a sulfonic acid group, Y representing a hydrogen atom, a carboxyl group or a sulfonic acid group, n representing 0 or 1, m representing an integer from 0 to 2;
said recording unit satisfying the relationship as defined by the following formula (2):
1.10xe2x89xa6Eop/Ethxe2x89xa61.90xe2x80x83xe2x80x83(2),
Eop representing the energy applied to the heater in order to cause the recording head to eject the liquid composition, Eth representing the minimum energy required by the recording head to eject the liquid composition.
In a further aspect of the invention, there is provided a method of prolonging a life span of a recording head to be used for an ink-jet recording method comprising a step of applying thermal energy to a liquid composition and causing it to be ejected through an orifice, said recording head being provided with a heater for applying thermal energy to the liquid composition, said heater having an outermost surface protection layer containing at least metal or oxide thereof;
said liquid composition comprising a liquid medium, an alkali metal ion and a compound expressed by the following general formula (1):
H2Nxe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94Xxe2x80x83xe2x80x83(1),
X representing a carboxyl group or a sulfonic acid group, Y representing a hydrogen atom, a carboxyl group or a sulfonic acid group, n representing 0 or 1, m representing an integer from 0 to 2;
said method satisfying the relation as defined by the following formula (2):
1.10xe2x89xa6Eop/Ethxe2x89xa61.90xe2x80x83xe2x80x83(2),
Eop representing the energy applied to the heater in order to cause the recording head to eject the liquid composition, Eth representing the minimum energy required by the recording head to eject the liquid composition.
As pointed out earlier, the inventors of the present invention found that, when ink containing alkali metal ions (sodium ions, lithium ions or the like) is used with the above described bubble jet method to produce a large volume of printed paper, the surface of the outermost surface protection layer of the heater (heat generating resistor) of the recording head can become dissolved. Thus, the inventors of the present invention paid research efforts for finding a compound that can effectively suppress the possible dissolution of the outermost surface protection layer of the heater in the presence of alkali metal ions and came to find that a compound expressed by general formula (1) is highly effective. The present invention is based on this finding.
According to the invention, the possible dissolution of the outermost surface protection layer containing at least metal or oxide thereof of the heater can be effectively suppressed. While it is not clear why the use of such ink provides the effect of suppressing the possible dissolution of the outermost surface protection layer of the heater, the inventors of the present invention presume the effect arises in a manner as described below.
Since the compound expressed by general formula (1) and contained in ink as indispensable ingredient is an acid having an amino group, ampho-ions expressed by formula (i) and (ii) exist in aqueous solution of the compound;
H3N+xe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94COOxe2x88x92xe2x80x83xe2x80x83(i)
and
H3Nxe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94SO3xe2x88x92xe2x80x83xe2x80x83(ii),
(where Y in the formulas (i) and (ii) represents a hydrogen atom, a carboxyl group or a sulfonic acid group, n represents 0 or 1 and m representing an integer from 0 to 2).
If alkali metal ions and hydroxide ions as counter ions thereof exist, the hydroxide ions are neutralized by the ampho-ions (i) and (ii) so that the alkalinity of the solution is lessened near the heater so that the possible dissolution of the metal and/or the metal oxide of the surface of the outermost surface protection layer is suppressed. When sodium hydroxide is used as hydroxide of alkali metal, reactions expressed by formulas (I) and (II) below will take place.
H3N+xe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94COOxe2x88x92+Na++OHxe2x88x92xe2x86x92H2Nxe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94COOxe2x88x92Na++H2Oxe2x80x83xe2x80x83(I)
H3N+xe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94SO3xe2x88x92+Na++OHxe2x88x92xe2x86x92H2Nxe2x80x94(CHY)nxe2x80x94(CH2)mxe2x80x94SO3xe2x88x92Na++H2Oxe2x80x83xe2x80x83(II)
Furthermore, as a result of the researches of the inventors of the present invention, it was found that the effect of preventing the possible dissolution of the outermost surface protection layer is boosted by the synergetic effect of the ink and the metal when tantalum and/or tantalum oxide are used respectively for the metal and/or the metal oxide.
Additionally, the effect of preventing the possible dissolution of the outermost surface protection layer containing metal and oxide thereof and arranged on the heater is further boosted when the relationship as defined by the formula below is satisfied;
xe2x80x831.10xe2x89xa6Eop/Ethxe2x89xa61.90,
where Eop represents the amount of energy applied to the heater per unit time in order to cause the recording head to eject the liquid composition and Eth represents the minimum amount of energy required by the recording head per unit time to eject ink.
According to U.S. Pat. No. 5,462,590 (Japanese Patent Application Laid-Open No. 6-220386), kogation of ink to the heater and any possible malfunction of the heater can be avoided by using ink containing alkali metal ions and a primary amine compound with the ink-jet method described in the patent document. However, U.S. Pat. No. 5,462,590 (Japanese Patent Application Laid-Open No. 6-220386) does not describe anything about the amount of energy to be applied to the heater per unit time. Furthermore, in view of U.S. Pat. No. 6,048,390 (Japanese Patent Application Laid-Open No. 10-36735) filed after U.S. Pat. No. 5,462,590 (Japanese Patent Application Laid-Open No. 6-220386) and describing that the use of primary amine is not desirable and the present invention can be used with any ink regardless of the coloring material contained therein, it may be safe to presume that the energy applied to ink in U.S. Pat. No. 5,462,590 (Japanese Patent Application Laid-Open No. 6-220386), if such energy needs to be defined, is out of the scope of the present invention.